Photo-detectors are used in imaging systems for medical, security and industrial applications. One particular medical application of photo-detectors is in computed tomography (CT) systems.
In a typical CT system, an X-ray source with a fan-shaped X-ray beam and a two-dimensional radiation detector array are assembled on a mechanical support structure, known as a gantry. In use, the gantry is rotated around an object to be imaged in order to collect X-ray attenuation data from a constantly changing angle with respect to the object. The plane of the gantry rotation is known as an imaging plane, and it is typically defined to be the x-y plane of the coordinate system in a CT system. In addition, the gantry (or more typically the object) is moved slowly along the z-axis of the system in order to collect x-ray attenuation data for a required length of the object. Examples of current CT systems are discussed in U.S. Pat. Nos. 6,144,718 and 6,173,031.
The radiation detectors of current state of the art CT systems consist of a two-dimensional array of rare earth metal based scintillators and a corresponding two-dimensional array of silicon photodiodes. Both the scintillator crystals and the photodiodes are manufactured in two-dimensional arrays, which are then optically coupled to one another during detector manufacturing.
A typical state of the art detector array is shown in FIG. 1. A typical detector consists of an array of 16 rows and 16 columns of individual detector elements, i.e. 256 elements in total. Columns are organised in the z direction. The construction of the detector is well-known in the art. The array of detectors is generally illustrated in FIG. 1 by reference numeral 2. The z direction or z-axis is also shown in FIG. 1. The elements in rows are in the imaging plane, and produce sets of data known as ‘slices’. In a medical CT machine, for example, each slice image corresponds to a two-dimensional X-ray image of a thin slice of a human body as seen in the direction of the body axis and the machine z-axis.
In CT imaging systems, the size of the detector in the imaging plane is increased by placing individual detector arrays, such as the array shown in FIG. 1, adjacent to each other to thereby increase the size of the detector in the imaging plane. An edge 4 of the detector of FIG. 1 may be placed alongside a corresponding edge of a corresponding detector array, and thereby a larger area can be built up.
A key trend in the CT industry is to build CT machines with more detector elements in order to collect more X-ray attenuation data for each gantry rotation and therefore to speed up the measurements, to improve the accuracy of the measurements, and to decrease patient radiation dose in medical applications. An increase in the number of detector elements similarly may have advantages in other imaging applications, and is not restricted to medical or CT systems.
In current CT detector constructions, a major limiting factor in providing more detector elements is the need to readout the electrical signals from the individual photo-detectors of the detector array. In the current art, the readout of these signals is facilitated by manufacturing very narrow metal lines (typically 5 to 20 μm) on top of the photo-detector chip, between the active photo-detector elements. A single metal line carries the signal of one photo-detector to the edge of the photo-detector chip, in the z direction, to an area which is specially reserved for the purpose of connecting the signals from the photo-detectors by wire bonding to a substrate placed beneath the photo-detector chip or to a multiplexing or signal processing ASIC chip. Using this method, there is a physical limitation on the size of a photo-detector array that may be manufactured. The number of electrical elements at the chip edge is limited, and this limits the number of photo-detector elements which can be connected. The detector cannot get larger in the z-direction in particular.
This is illustrated by FIG. 1. The photo-detector array 2 is provided with an area 6 and 8 either side of the array in the z direction, which areas provide for connection to a respective set of electrical wires, 10 and 12. The signals from the photo-detector array may be multiplexed or processed in integrated electronics chips or ASICs located in areas 6 and 8 before the signals are connected to electrical wires, 10 and 12. Because of the need to accommodate the physical wires and their connections, the number of photo-detectors in an array is limited. In particular, it is not possible to add further photo-detectors in the z direction. The physical wires 10 and 12 prevent any expansion of the photo-detector array in the z direction, such that additional photo-detector arrays cannot be added in the z direction. That is, although photo-detectors can be joined together side-by-side, in the horizontal direction in FIG. 1, they cannot be joined to top-to-bottom, in the vertical direction. This is because of the need to connect the wires 10 and 12 at the top and bottom.
A photo-detector with the possibility of expansion in the z direction is known as a ‘tileable’ detector. In order to provide a tileable detector, it is necessary to make the electrical connections to each photo-detector without wiring the photo-detectors to the photo-detector chip edge. If this can be achieved, there is no limit to the growth of the photo-detector array and consequently the number of photo-detector elements.
One solution to the problem of achieving a tileable detector is suggested in U.S. Pat. No. 6,396,898.
Embodiments of the present invention aim to address one or more of the above problems and to provide an improved photo-detector array.